The invention is related to a fluidized catalytic cracking process which process comprises contacting a hydrocarbon feedstock with a fluidized particulate catalyst in a reaction zone wherein a hydrocarbon product is prepared and wherein coke accumulates on the catalyst to become a spent catalyst. The coke is removed in a regenerator by means of combustion and the regenerated catalyst is reused in the reaction zone.
Despite the long existence of the fluidized catalytic cracking (FCC) process, techniques are continually sought for improving product recovery both in terms of product quantity and composition, i.e. yield and selectivity. A facet of the FCC process that has received attention is the recovery by means of stripping of adsorbed products from the spent FCC catalyst. Improvement in the recovery of hydrocarbons from spent catalyst directly improves yields. Aside from increasing hydrocarbon recovery, reducing the carryover of hydrocarbons into the regeneration zone improves the overall heat balance of the FCC unit. Hydrocarbon that enters the regeneration zone releases additional high temperature heat as it burns in the oxygen atmosphere. Any additional heat release in the regenerator may interfere with the regenerator operation by unduly raising temperatures in the regeneration zone or requiring cooling methods to maintain a suitable temperature.
The processing of increasingly heavier feeds and the tendency of such feeds to elevate coke production makes the control of regenerator temperatures difficult. The increase in coke on spent catalyst results in a larger amount of coke being burned in the regenerator per ton of catalyst circulated. Heat is removed from the regenerator in conventional FCC units in the flue gas, and principally in the hot regenerated catalyst stream. An increase in the level of coke on spent catalyst will increase the temperature difference between the reactor and the regenerator, and the regenerated catalyst temperature. A reduction in the amount of catalyst circulated is, therefore, necessary in order to maintain the same reactor temperature. However, the lower catalyst circulation rate required by the higher temperature difference between the reactor and the regenerator will lower hydrocarbon conversion.
By improving the efficiency of the stripping process less adsorbed hydrocarbons in addition to the fixed coke will be present on the catalyst supplied to the regenerator, resulting in a decrease in the temperature difference between the reactor and the regenerator. Various possible methods have been developed to increase the efficiency of the stripping process. For example European patent specification EP-A-702077 describes a more efficient stripping process. In this process the catalyst is first stripped in a conventional dense phase stripping zone followed by stripping in a dilute phase stripping zone. The thus stripped catalyst, after being separated from the stripping medium, is sent to the regenerator. In the dilute phase stripping zone the spent catalyst is mixed with some hot regenerated catalyst resulting in that the stripping temperature and thus the stripping efficiency is increased.
European patent specification EP-A-322276 describes a comparable process as described in EP-A-702077. As an additional feature oxygen is present in the lift gas of the dilute phase stripping zone.
U.S. Pat. No. 3,856,659 discloses a FCC process wherein part of the spent catalyst is mixed with part of the regenerated catalyst. This mixture is contacted with steam in a dense fluidized bed. The catalyst mixture is subsequently supplied to a riser reactor in which reactor the catalyst mixtures undergoes at least partial regeneration by burning off carbonaceous deposits.
U.S. Pat. No. 3,894,934 discloses a FCC process comprising a first and second elongated riser reactor, a dense phase fluidized stripping zone and a catalyst regenerator. The process comprises a step wherein part of the catalyst obtained directly after separation from a hydrocarbon product as obtained in the first riser reactor is supplied to the second riser reactor. To this second riser reactor a hydrocarbon feedstock and part of the regenerated catalyst is also supplied.
The following process according to the invention provides an even more efficient method of stripping the spent catalyst: Fluidized catalytic cracking process which process comprises contacting a hydrocarbon feedstock with a fluidized particulate catalyst in a reaction zone wherein a hydrocarbon product is prepared and wherein coke accumulates on the catalyst to become a spent catalyst and which process comprises of the following steps:
(a) separating the hydrocarbon product from the spent catalyst by means of one or more gas-solid separation steps;
(b) stripping the spent catalyst in a dense phase fluidized stripping zone by introducing a stripping medium in the lower portion of the stripping zone;
(c) introducing part of the spent catalyst obtained in step (b) to a regeneration zone wherein the coke is removed from the catalyst by means of combustion;
(d) introducing the remaining part of the spent catalyst obtained in step (b) and part of the hot regenerated catalyst obtained in step (c) into a lower portion of an elongated dilute phase stripping zone;
(e) introducing a stream of a stripping medium into the lower portion of the dilute phase stripping zone to contact the resulting mixture of spent catalyst and regenerated catalyst therein;
(f) passing a stream of the spent catalyst mixed with the hot regenerated catalyst and stripping medium upwardly in the dilute phase stripping zone under dilute phase stripping conditions to an upper portion thereof;
(g) separating substantially all of the spent catalyst and regenerated catalyst from the effluent of step (f) and introducing the separated catalyst to the dense phase stripping zone of step (b);
(h) passing the remaining part of the hot regenerated catalyst obtained in step (c) to the reaction zone to be contacted with the hydrocarbon feedstock.
It has been found that with the process according to the invention a more efficient stripping method is obtained because a higher temperature in the dense phase stripping zone can be achieved. This because the catalysts leaving the dilute phase stripping zone, having a more elevated temperature than the catalyst leaving the reaction zone, are also supplied to the dense phase stripping zone. Because a more efficient stripping process is achieved less absorbed hydrocarbons will enter the regeneration zone resulting in a regenerated catalyst having a lower temperature than possible with the prior art processes. This gives the operator of the FCC process the possibility to increase the amount of regenerated catalyst to be used in the reaction zone, resulting in a higher conversion while maintaining the quality of the products thus obtained. Further advantages of the invention will become apparent from the below detailed description of the invention.
The hydrocarbon feedstock include conventional FCC feeds and higher boiling or residual feeds. The most common of the conventional FCC feeds is a vacuum gas oil which is typically a hydrocarbon material having a boiling range of from 350-530xc2x0 C. Vacuum gas oils are the distillate fraction obtained by vacuum distillation of a atmospheric residue fraction, which are in turn obtained from distilling a crude petroleum feedstock at atmospheric pressure. The process according to the present invention is especially suitable for processing heavier hydrocarbon feedstocks than vacuum gas oils like for example the atmospheric residue fraction directly.